Method and apparatus for storage tank leak detection

ABSTRACT

An intrinsically safe, precise, and cost effective storage tank leak detection system. Embodiments include a highly precise quartz crystal type pressure transducer, secured in a vacuum and held at a constant temperature. Said transducer is used in combination with data correction and regression techniques to yield a storage tank leak detection system with an extremely low leak detection threshold. The storage tank leak detection system is thought to be most useful for detecting very small leaks in very large storage tanks.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed towards a method and apparatus for providing a safe, precise, and cost-effective storage tank leak detection system and more particularly, to a method and apparatus wherein the containment integrity of a storage tank is determined by mass measurements of the stored product.

2. Background Information

Storage tanks play a vital role in today's economy. The economy, on a global scale, depends on the proper function of these tanks as they are prevalent in several industries and virtually every geographical region in the world. In light of the vital role these storage tanks play, the integrity of the tanks is placed at a premium. That is, storage tank owners are willing to invest huge sums of money in both the maintenance and inspection of such tanks.

These tanks come in all shapes and sizes, are found both below and above ground, and are used to store a wide-range of materials. Storage tank capacities range from hundreds to millions of gallons and are used to store a staggering assortment of products; these storage tanks are commonly used to store hazardous material.

As one could imagine, there are a wide range of problems associated with maintaining storage tank integrity, particularly with above ground storage tanks. Given the enormous dimensions of above ground tanks, the corrosive products contained within the tanks, the incredible mass of the stored product, and the extreme weather conditions the tanks are subjected to; it is plain to see that above ground storage tank leaks are an all-to-common problem. Using the United States Environmental Protection Agency leak detection threshold criteria of 0.05 gallons per hour in a 10,000-gallon underground tank, that threshold would equate to a 15 gallon per hour detection level in an 80,000 barrel above ground tank. Given the limited number of systems capable of meeting the EPA's underground storage tank leak detection threshold and the added difficulties associated with above ground tanks, the difficulty in protecting against and detecting leaks is easily seen.

However, the recognized difficulty in preventing storage tank leaks does not mitigate the duties or liabilities imposed on responsible parties. Tremendous environmental and economic consequences and the threat of litigation and clean up costs associated with storage tank leaks force responsible parties to invest large sums of money in the maintenance and inspection of the tanks. Tank inspections are costly with respect to the amount of money spent, the danger presented to the inspectors and the environment, and production downtime. In fact, these inspections often remove a tank from service for more than one month. The threat of liability also forces responsible parties to spend money unnecessarily for the maintenance of these tanks. Moreover, liability does not end with litigation and clean-up costs.

Currently, responsible parties are, in some countries, being incarcerated as a direct result of storage tanks leaks. These leaks have contaminated surrounding ground water, some of which serves as drinking water for local residents. As such, the facilities associated with such incidents have been shutdown until compliance with emissions regulations can be established beyond reasonable doubt. Such proof, in turn, is dependent on proof of reliable and sufficiently accurate detection systems and methods for proving such compliance. Each day the shuttered facilities remain inoperative adds to an already tremendous amount of money lost.

Prior to the present invention (to be described in detail hereafter), there are simply no known systems or methods by which the leak detection requirements can be met. Presently available leak detection systems lack detection thresholds low enough to detect leaks down to permissible upper leakage limits for above ground storage tanks.

Clearly, for the reasons set forth above, there is a dire and immediate need for the ability to determine, with far more precision than presently possible through use of presently available systems and methods, the presence and degree of leakage from above ground storage tanks, at least to the extent of proving compliance with applicable storage tank leakage regulations or statutes.

Comparison with Known Technologies in the Field

Storage tank leak detection systems are known in the art; however, these products are fraught with problems. The present systems are imprecise, or provide erroneous data for any or all of reasons including: the consistency of the soil acting as the tank's foundation, the temperature stratification of the in-tank product, extraneous noise sources, thermal expansion of the tank's contents, water table level, previous soil contamination, and/or tank shell dynamics.

Further, some detection devices can only be used when the storage tank is empty, and no known system or method ensures a comprehensive inspection of the tank. The most common form of such a system is “vacuum box testing;” however, this system is intended only for weld joints and is not usually applied to the entire tank bottom. Magnetic flux floor scanning is also used, but is not effective at examining the area of the floor surface close to the surface walls or where there are physical obstructions. Ultrasonic detection is used, but this is only effective for small areas of the surface. Gas detection is also used, but the types of materials stored in the tank can obstruct this method.

Other common leak detection systems employ a level sensor. However, even large volume changes produce only small level changes, as the cross-sectional area of the liquid surface in these tanks is very large. This, combined with differential expansion and temperature change of the stored liquid and its vapor, make this type of detection system inconsistent and very nearly worthless.

Finally, mass measurement detection systems are known in the art. However, the presently available systems and associated methods are not capable of the precision, which is indicated above as crucial at the present time (and which, as described below, is afforded by the systems and methods of the present invention). Present mass measurement leak detection systems in the art are limited by tank shell variations resulting from temperature effects on tank shell plating. As such, known mass measurement detection systems are only sensitive enough to be used in smaller tanks, typically underground storage tanks. However, as will be seen in the specification to follow, the present invention overcomes tank shell variations and other shortcomings of presently known technology in this field through data collection and data correction apparatus, techniques and interpretation.

In light of the severe consequences of failing to detect significant storage tank leaks, presently not detectable through use of known systems or methods, there is a compelling need for a system and method by which one can detect very small leaks even in very large tanks, ideally in a safe and cost effective manner.

It would well serve those who are responsible for maintaining storage tank integrity to provide a safe, precise, and cost-effective detection system that does not depend on independent variables such as fluid temperature, fluid stratification, or tank stabilization, and may be used in an efficient manner thereby preserving industrial and environmental resources.

The system by its very nature is also a very effective means of accurately measuring the amount of fluid (product) added to or removed from a tank. Accordingly the system represents an attractive alternative to hand strapping and other volume based (through flow meter) product custody transfer measurement methods and apparatus. Where the accuracy of custody transfer measurements is of significance due to the quantities, commercial terms and value of the product this system can be directly employed to accurately measure the transfer amounts.

SUMMARY OF INVENTION

In view of the foregoing, it is an object of the present invention to provide a storage tank leak detection apparatus with a very low detection threshold that may be used in an efficient manner thereby preserving industrial and environmental resources.

It is another object of the present invention to provide an apparatus for safe storage tank leak detection

It is another object of the present invention to provide an apparatus for precise storage tank leak detection

It is another object of the present invention to provide an apparatus for cost-effective storage tank leak detection

It is another object of the present invention to provide an apparatus for non-intrusive storage tank leak detection

It is another object of the present invention to provide an apparatus for storage tank leak detection where the contents of the storage tank do not have to be removed

It is another object of the present invention to provide an apparatus for storage tank leak detection where no chemical additives are involved

It is another object of the present invention to provide an apparatus for immediate storage tank leak detection

It is another object of the present invention to provide an apparatus for conclusive storage tank leak detection

It is another object of the present invention to provide an apparatus for quantitative storage tank leak detection

It is another object of the present invention to provide an apparatus to determine whether head changes are directly attributable to leaks or other effects

It is another object of the present invention to provide an apparatus for storage tank leak detection that does not depend on fluid temperature changes

It is another object of the present invention to provide an apparatus for storage tank leak detection that does not depend on fluid stratification

It is another object of the present invention to provide an apparatus for storage tank leak detection that does not require tank stabilization time

It is another object of the present invention to provide an apparatus for storage tank leak detection that requires only minimal tank preparation

It is another object of the present invention to provide an apparatus for safe measurement of product custody transfers

It is another object of the present invention to provide an apparatus for cost-effective measurement of product custody transfers

It is another object of the present invention to provide an apparatus for precise mass based measurements of product custody transfers

It is another object of the present invention to provide an apparatus for measurement of product custody transfer that does not depend on fluid stratification.

It is another object of the present invention to provide an apparatus for highly accurate measurement of product custody transfer where flow meters are not available.

It is another object of the present invention to provide a method with a very low detection threshold that may be used in an efficient manner thereby preserving industrial and environmental resources.

It is another object of the present invention to provide a method for safe storage tank leak detection

It is another object of the present invention to provide a method for precise storage tank leak detection

It is another object of the present invention to provide a method for cost-effective storage tank leak detection

It is another object of the present invention to provide a method for non-intrusive storage tank leak detection

It is another object of the present invention to provide an apparatus for storage tank leak detection where the contents of the storage tank do not have to be removed

It is another object of the present invention to provide a method for storage tank leak detection where no chemical additives are involved

It is another object of the present invention to provide a method for immediate storage tank leak detection

It is another object of the present invention to provide a method for conclusive storage tank leak detection

It is another object of the present invention to provide a method for quantitative storage tank leak detection

It is another object of the present invention to provide a method to determine whether head changes are directly attributable to leaks or other effects

It is another object of the present invention to provide a method for storage tank leak detection that does not depend on fluid temperature changes

It is another object of the present invention to provide a method for storage tank leak detection that does not depend on fluid stratification

It is another object of the present invention to provide a method for storage tank leak detection that does not require tank stabilization time

It is another object of the present invention to provide a method for storage tank leak detection that requires only minimal tank preparation

It is another object of the present invention to provide a method for safe measurement of product custody transfers

It is another object of the present invention to provide a method for cost-effective measurement of product custody transfers

It is another object of the present invention to provide a method for precise mass based measurements of product custody transfers.

It is another object of the present invention to provide a method for measurement of product custody transfer that does not depend on fluid stratification.

It is another object of the present invention to provide a method for highly accurate measurement of product custody transfer where flow meters are not available.

Embodiments and practice of the present invention provides a safe, extremely precise, and cost-effective solution to the problems mentioned above. Test results produced through practice of the present invention provide a determination of containment integrity, transfer quantities and in the event of leakage, and a precise volumetric leak rate, all at accuracy levels unprecedented in the art. Use of the present invention is not restricted by fluid type, fluid temperature, fluid level, or tank size.

The present invention obviates the need for physical inspection of storage tanks. As such, there is no need to drain, clean, or enter a to-be-inspected/monitored tank, therefore, neither inspectors nor the environment are exposed to the contents of the tank as a result of the evaluation. In addition to eliminating hazardous by-products associated with the draining/cleaning process, also avoided is the danger of transporting and storing the drained product.

System control techniques and data correction software provide for precision not possible with systems now known in the art. Finally, the systems and methods of the present invention do not require chemical additives to be mixed with the tank contents. As such, incidental spills and leaks of such agents are avoided altogether.

Practice of the present invention, in lieu of alternative, conventional technologies, is extraordinarily cost effective. Neither a tank structure, its foundation, nor the surrounding soil are disturbed as part of monitoring or inspection of tanks pursuant to the present invention. As such; set-up time and capital investment costs are minimized.

Because practice of the present invention eliminates manual inspection of a tank, it is non-intrusive. Therefore, operation of the tank is not hindered, and production downtime is altogether avoided.

Costs related to the handling, transport, disposal, or storage of removed hazardous material (required by, among other methods, any method requiring internal, physical inspection of a tank) are likewise avoided.

The unprecedented untiltiy afforded by the present invention, arising from the unprecedented accuracy of the system of the present invention, is the product of substantially improved sensitivity (compared to prior technologies) with which the present system is able to measure mass variations.

The present invention, by employing a combination of techniques and components not known in the art, provides a leak detection threshold that is much lower than any known device. This is important because, the lower the leak detection threshold level of such a system, the more effective it will be at detecting leaks.

One feature of the present system involves its maintaining the mass measurement component's temperature at a substantially constant level during an entire measurement process. Further, certain features of the system correct errors in the collected data which are attributable to storage tank shell dynamics, as well as to the inherent imprecision in the mass measurement devices. This data correction process will be discussed in detail in the specification to follow.

The systems and methods of the present invention overcome tank shell variations (a major “weak link” in mass measurement-based systems) through data collection and data correction techniques. First, data is collected through use of a quartz crystal type pressure transducer (the specifications and use of this transducer will be explained in more detail in the Detailed Description of the Preferred Embodiment). A PLC connected to the pressure transducer, records pressure data over a period of time (preferably one to five nights). The atmospheric temperature and barometric pressure are recorded and precisely analyzed to calculate any changes in the mass of the fluid within the tank. The atmospheric temperature is recorded and precisely analyzed to correct for indicated changes in the observed mass which arise only from tank shell temperature variations. The corrected data is regressed to give a line slope that is converted to a leak rate of volume over time (the system usually being configured for reporting in gallons per hour).

Data generated by the transducer is, according to the preferred mode of the present invention, collected on a 24-hour basis. Only data containing a sufficiently low amount of extraneous noise is analyzed. Such data is, accordingly, usually obtained at nighttime and during fair weather conditions. Such selective data gathering eliminates, among other adverse factors, the effect of the sun's radiant energy on the surface area of the tank, which may adversely affect the mass measurement of the stored product. Also, data correction software accounts for the coefficient of thermal expansion for any given storage tank construction material. The nighttime data is corrected for atmospheric conditions and variations in the tank shell. These measurements and corrections allow the system to repeatedly achieve the stated accuracy in real world conditions on a routine basis.

For even greater precision, the leak detection system of the present invention provides for an independent barometric measuring means to constantly record the barometric pressure during the data collection process. This independent barometric pressure measuring means, used in combination with data correction software, corrects any zero drift associated with the individual pressure transducer. That is, this system corrects for the inherent error present in any transducer when that transducer deviates from its initial reference pressure.

Practice of the present invention involves use of a combination of precise mass measurement components, including a highly precise quartz crystal type differential pressure transducer, in a vacuum-sealed canister. This canister is then lowered to the bottom surface of a storage tank. A differential reference is placed just above a liquid surface. The pressure, measured at the tank floor (“tank bottom pressure”) and atmospheric and vapor pressure measured just above the liquid surface, is recorded by the above-referenced micro sensitive differential pressure transducer, recorded on a real time basis and post processed using a data analysis routine to accurately calculate any changes in the mass of fluid contained within the tank to determine if there is a loss. The present system, using the specified transducer, and when used in the manner and with the data interpretation described herein, is capable of detecting above ground storage tank leaks at a threshold of less than 0.9 gallons per hour with a probability of detection of 95% in a 120 foot diameter tank—far more accurate than possible with any presently available quantitative leak detection system. This, quantitatively, amounts to detecting pressure differentials equivalent to less than 1/10,000th inch of water column pressure, a tolerance level necessary to achieve such detection thresholds.

The method and apparatus of the present invention provides a safe and effective way to detect very small leaks in very large tanks. Particularly, the present invention provides a tremendous improvement in accuracy and leak detection threshold, allowing its operators to achieve greater results than presently thought possible.

Thus, in satisfaction of the above objects, an embodiment of the present invention provides systems and methods for solving each of the stated problems with presently available storage tank leak detection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Annex A is a printout of the industrial controller program source code referred to herein as the PLC program.

FIG. 1 is a block diagram depicting the general layout of the present leak detection system.

FIG. 2 is an elevational, sagital cross sectional view of the canister (“protective enclosure means”) of the leak detection system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the drawings and the description that follows, FIGS. 1 and 2 generally depict a preferred configuration and constituency of a storage tank leak detection system according to the present invention, which system is generally designated by the reference number 10.

The preferred embodiment of the present invention includes a vacuum-sealed canister 12, which houses and protects a plurality of mass measurement components and system control components. In the preferred embodiment, vacuum-sealed canister 12 is made of a substantially non-corrosive metal (aluminum, for example), however, any material that is corrosion resistant and offers sufficient protection to the components enclosed is adequate for use with the present invention.

Canister 12 is to be directly immersed in storage tank 60 and it to rest substantially at a storage tank bottom surface 62. Canister 12 further contains vacuum seal nozzle 14 and transducer high side aperture 20. Vacuum seal nozzle 14 allows communication means to pass from the inside of the canister to the outside of the canister while maintaining the integrity of the vacuum inside the canister. Vacuum nozzle 14 further contains transducer low side aperture 13.

At its proximate end, canister hose 15 forms a fluid tight seal with vacuum seal nozzle 14. Extending from vacuum seal nozzle 14, canister hose 15 passes through storage tank top surface access 65 to an area outside of the class I region of storage tank 60 (class I region refers to the National Electric Code designated hazardous areas in which only power wattage levels of less than certain prescribed levels may be introduced). Canister hose 15 serves as a conduit for communication means extending though vacuum nozzle 14 and as an atmospheric reference in its service as a barometric pressure measuring means reference hose. Canister hose 15 allows transducer low side aperture 13 to be directly exposed to atmospheric pressure above the tank fluid while maintaining a fluid tight seal with vacuum seal nozzle 14 thereby preserving the integrity of the vacuum of canister 12.

Contained within vacuum-sealed canister 12 is differential pressure transmitter 22. In the preferred embodiment, differential pressure transmitter 22 is comprised of a highly precise quartz crystal type pressure transducer 24 and a highly precise quartz crystal temperature transducer 25. Transducer 24 contains an oscillating quartz crystal and has a pressure resolution of 1×10-8, as known in the industry. The ultimate resolution achievable with a transducer is limited by its noise level.

System 10 greatly reduces noise thereby increasing the resolution of transmitter 24. In system 10, transmitter 22 has been modified from its original configuration so that it may be directly immersed in storage tank 60. This modification has eliminated dependence on any bubbler unit (thereby eliminating noise associated with such units) as required by other products. As will be further described in this section, transducer 24 is held at a constant temperature and secured in vacuum to further reduce noise.

Quartz crystal type pressure transducer 24 is further comprised of transducer low side 26. Transducer low side 26 is a differential reference that receives the atmospheric pressure value at the liquid surface. Transducer low side tube 28 forms an air tight seal at it proximate end with transducer low side 26 and extends though the vacuum of canister 12 where it forms an air tight seal at its distal end at transducer low side aperture 13 of vacuum seal nozzle 14. Transducer low side tube 28 allows transducer low side 26 to receive the atmospheric pressure from the reference point at the liquid surface while allowing canister 12 to remain in vacuum.

Quartz crystal type pressure transducer 24 is further comprised of transducer high side 30. Quartz crystal type pressure transducer high side 30 is a pressure reference point, which measures the sum of the atmospheric (barometric) and hydrostatic pressure at tank bottom surface 62. Transducer high side 30 contains a protruding transducer high side tube 32.

In the preferred embodiment, transducer high side tube 32 is filled with a pressure-sensing liquid and extends through transducer high side aperture 20 where it is ported to the product contained in tank 60. Transducer high side tube 32 is surrounded by tube fitting 34. In the preferred embodiment, tube fitting 35 slides along high side tube 32 and forms a fluid tight seal at high side aperture 20. Tube fitting 35 allows high side tube 32 to extend through high side aperture 20 while maintaining the integrity of the vacuum of canister 12.

Transducer 24 measures the pressure differential between the transducer low side 26 and transducer high side 30 to arrive at the pressure exerted by the mass of the stored product. Transmitter 22, communicating digitally, then sends this processed information to PLC 34. This data is transmitted along data transfer means 23. In the preferred embodiment, data transfer means 23 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well. Data transfer means 23 extends from the output of differential pressure transmitter 22 through vacuum seal nozzle 14 and continues, separated from storage tank's 60 contents by canister hose 15, to the PLC 34.

Also contained within canister 12 is fluid temperature transmitter 37. Fluid temperature transmitter 37 is mounted and thermally isolated from other canister 12 components so as to provide an accurate measure of the fluid temperature in tank 60. This data is transferred along data transfer means 39. Data transfer means 39 extends from the output of fluid temperature transmitter 34 through vacuum seal nozzle 14 and continues, separated from storage tank's 60 contents by canister hose 15, to the PLC 34. In the preferred embodiment, data transfer means 39 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.

Quartz crystal temperature transducer 25 serves as a part of a temperature regulation scheme used to keep transducer 24 at a constant temperature during the data gathering process. Quartz crystal temperature transducer 25 communicates digitally via transmitter 22 with the PLC 34. This data is transmitted along data transfer means 23. In the preferred embodiment, data transfer means 23 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.

In the preferred embodiment the PLC, through a control loop, generates an output which activates resistive heater 36 which in combination with heat sink 38 regulates the temperature of transmitter 22. While the above temperature regulating scheme has been described with reference to one embodiment, one could easily imagine other temperature regulation schemes that would work equally as well.

The use of this temperature regulation scheme to hold transmitter 22 at a constant temperature further increases the precision of the apparatus. The absolute temperature at which transmitter 22 is maintained is not critical, rather constancy of temperature affects the integrity of the subject measurements. As a matter of practicality and economy, temperature of transmitter 22 is maintained, according to the presently preferred mode of the present invention, at a temperature of approximately 10° F. above the ambient temperature of the product (oil or gasoline, for example) in tank 60. If, for example, the product is at 50° F., transmitter 22 is maintained at 60° F., if the product is at 90° F., transmitter 22 is maintained at 100° F., and so forth.

Also contained within canister 12 is barometric pressure measuring means 40. Barometric measuring means 40 serves as an independent reference for true atmospheric pressure. In the preferred embodiment, barometric pressure measuring means 40 may be any standard barometer that sends signals to be processed by PLC 34. Barometric measuring means 40 is very useful for increasing the precision of system 10. The present invention employs barometric measuring means 40 to serve as an independent measure of true atmospheric pressure thereby allowing for data correction over any extended period of time. As will be discussed in this section, data correction using values taken from barometric pressure measuring means 40 greatly increases the precision of the current invention.

Barometric measuring means tube 42 forms an air tight seal at its proximate end with Barometric measuring means 40 and tees into tube 28 at its distal end. Barometric measuring means tube 42 allows barometric measuring means 40 to receive the barometric pressure from the reference point at the surface of liquid within storage tank 60, while allowing the interior of canister 12 (with transmitter 22 installed therein) to remain in vacuum so as to substantially eliminate any environmentally-effected variations in instrument performance). Data transfer means 43 extends from the output of barometric pressure measuring means 40 through vacuum seal nozzle 14 and continues, separated from storage tank's 60 contents by canister hose 15, to the PLC 34. In the preferred embodiment, data transfer means 43 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.

A measurement of ambient temperature is made by temperature transmitter 50. Ambient temperature transmitter 50 is mounted outside of the class I region of storage tank 60 so as to provide an accurate measure of the ambient temperature of the air surrounding tank 60. This data is transferred along data transfer means 53 to the PLC 34. This is very useful for increasing the precision of system 10.

Although not necessary, the PLC 34 and the computer 70 are typically housed in a common enclosure, such as field unit 100. In accordance with the described routines to follow and the exemplary computer code depicted in Annex A attached hereto and incorporated herein by reference, the PLC processes data received from transmitter 22, fluid temperature transmitter 37, ambient temperature transmitter 50 and barometric pressure measuring means 40. The PLC also controls the temperature transmitter 22 by means of resistive heater 36 and heat sink 38. The PLC 34 communicates with man-machine interface computer 70 by data transfer means 72. In the preferred embodiment, data transfer means 72 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.

Man-machine interface computer 70 provide the means whereby the operator interacts with the system to: log data, monitor system operation, enter temperature set point, record job specific physical site and client data, collect and process custody transfer data, trouble shoot detected errors and enter password keys to authorize data collection.

The software commences operation with the initialization of data collection at the tank bottom, along with the atmospheric and environmental conditions. Data is automatically collected via industrial computer controlled programming over some length of time, preferably 36 to 60 hours. The length of the test is dependent on tank size and site weather conditions

As will be discussed and illustrated hereafter, remote computer 80 contains software that performs linear regressions of data downloaded from the man-machine interface computer 70. This regression detects minuscule changes in the mass of the stored product, thereby indicating the presence of the smallest of leaks. As the compilation of data grows, the more precise the regression becomes. The post processing module and software of remote computer 80 is independent of the PLC 34 and the man-machine interface computer 70.

There are three software programs or modules involved with the storage tank leak detection system of the present invention: The PLC program, the man-machine interface computer program and the post processing program operated on remote computer 80. The PLC program is performed by the PLC 34 and is responsible for obtaining (subroutine Measure) data from transmitter 22, controlling the temperature of transmitter 22 (subroutine Temp_Ctrl), obtaining transmitter 22 differential pressure and temperature (subroutine Measure), and backup data storage. The data acquired by the PLC program is stored within the PLC 34 in non-volatile memory.

The PLC program interrogates the differential pressure transmitter (transmitter 22) via a serial connection. The pressure read from transmitter 22 is the difference in pressure read from transducer low side 26 and transducer high side 30. That pressure value is modified by two additional variables in order to improve the accuracy of the reading. The post processing program performs measured head corrections for a) tank shell temperature changes based on measurements of ambient temperature and fluid temperature and b) atmospheric pressure changes which otherwise would skew the data interpretation. This post processing is intended solely to detect variations of contents of storage tank 60 due to leakage and eliminate variations due to environmental changes. Any change in tank diameter is accommodated in the calculations thus properly attributing substantially all variations in differential pressure to variations in the content of storage tank 60, such as through leakage.

The PLC subroutines Measure and Tx_Cmplt_Int obtain pressure readings and transducer temperature readings from transmitter 22. This performed every one minute.

Subroutine Temp_Ctrl controls transducer 24 temperature, this is performed as follows: the operator entered temperature set point is compared to the digitally communicated transducer temperature, the difference or error is then used to establish the resistive heater output.

The remaining recorded variables are also obtained on a one minute time frame. This is accomplished in the Measure subroutine.

Finally, the PLC program is responsible for data storage. This is accomplished in subroutine Record. One record per minute is stored. The organization of the data is by date and time. The record for every minute will include: (1) the differential pressure representing the hydrostatic pressure produced by the fluid mass (as a floating-point number, IEEE 32 bit format), (2) the barometric pressure (as a ×1000-16 bit integer), (3) the ambient temperature (as ×100-16 bit integer), (4) the fluid temperature (as ×100-16 bit integer), (5) the transducer temperature (as ×100-16 bit integer).

The system features a man-machine interface computer 70 to allow direct operator interaction with the unit, to accept the operator's transducer temperature setpoint, to allow the use to graphically visualize the differential pressure data collection via a time based chart, to allow the operator to determine the state of the machine, to allow the operator to trouble-shoot any system recognized errors, to provide a second (primary) means for logging the collected data, to allow the operator to authorize data collection by way of entry of a predetermined “run key”, to allow the operator to record client, location, tank and fluid physical characteristics, to allow the operator to initiate and conclude mass based fluid custody transfer measurements.

The third software program of the storage tank leak detection system of the claimed invention is the post processing program. Remote computer 80 performs this program. Linear regression of logged data, is performed as follows. The data files created by the PLC program are read in, these include time stamped values for hydrostatic pressure created by the fluid mass, ambient temperature, fluid temperature, transducer temperature, atmospheric pressure and atmospheric pressure, along with the tank diameter and the specific gravity of the tank contents. Three graphical presentations of hydrostatic pressure (fluid mass) versus time are created, one of the raw hydrostatic pressure data, one of the tank shell temperature corrected hydrostatic pressure data and one of the tank shell temperature corrected hydrostatic pressure data with atmospheric pressure corrections. Tank shell temperature corrections are made on the basis of the coefficient of thermal expansion of the construction material of the tank and the calculated tank shell temperature. The calculated tank shell temperature is derived from a combination of the fluid temperature and the ambient temperature at a ratio entered by the software operator, typically in the range of 0.2 to 1.0. The selection of the ratio to apply is dependent upon tank physical conditions such as the presence of external insulation, weather conditions including rain, wind and humidity and the tank contents (the appropriateness of the selected ratio is further verified by the method presented below). Atmospheric pressure corrections are made based on the application of a barometric correction coefficient applied to the hydrostatic pressure data (this barometric correction coefficient is determined empirically through the observation of head variations as a function of barometric pressure changes in an otherwise leak free and well insulated test tank during initial system commissioning). The operator selects data periods, typically consistent nighttime periods where the data is relatively free from the significant effects of solar radiant energy, rain and other significant weather events and the software performs linear regressions of the three hydrostatic pressure data series over these periods. The resultant slope of the linear regression of tank shell temperature corrected hydrostatic pressure with barometric pressure corrections is converted to a leak rate on the basis of the tank diameter and the specific gravity of the tank contents. In order to validate the results and identify any hydrostatic pressure changes that are inconsistent with a leak-free tank the software performs a validation of the data by calculating the theoretical tank shell temperature on the basis of the measured hydrostatic pressure change throughout the duration of the test. This theoretical tank shell temperature is presented graphically superimposed upon the measured fluid temperature and the measured ambient temperature data and manually scaled to align the first nights theoretical tank shell temperature to the region bounded by the measured ambient temperature and the measured fluid temperature for the same time period. Through direct visual interpretation the operator can identify whether or not the theoretical tank shell temperature is consistent with the measured ambient and fluid temperature for all data throughout the duration of the test and similarly verify the appropriateness of the operator selected fluid and ambient temperature ratio. More specifically if the theoretical tank shell temperature calculated on the basis of the measured head change does not fall within the region banded by the ambient and fluid temperature on subsequent nights the excursion is of interest and observed for linearity. The operator can apply a simulated leak rate to verify the linearity of the excursion and to confirm the leak rate measured through linear regression of the hydrostatic pressure data. This method allows the operator to correlate changes in measured head with changes in ambient and fluid temperature and if the correlation is strong validate the measured head data. Where the correlation is not strong and the excursion from night to night is not linear the change in head can be attributed to other outside influences and thereby the use of irrelevant data avoided (these could include effects such as the in tank probe settling in tank sediment, tank pumping operations, etc).

It is believed that, while safe and efficient, the present device will obviate significant inconvenience and provide substantial utility to those who wish to detect leaks in storage tanks. Specifically, the present device will allow very small leaks to be detected in very large storage tanks in a consistent and cost-effective manner.

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limited sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the inventions will become apparent to person skilled in the art upon the reference to the description of the invention. It is therefore contemplated that the appended claims will cover such modification that fall within the scope of the invention. 

1. A system for detecting and quantifying changes in the mass content of liquid storage containers comprising: differential pressure sensor means configured for receiving input representative of first and second pressure parameters and for transmitting data representative of differential between said first and second pressure parameters to computing means; high pressure measurement means configured for measuring a first pressure parameters in a liquid medium residing with in a container, and engaged with said differential pressure sensor means for facilitating the sensing of said first pressure parameters by said differential pressure sensor means; low pressure measurement means configured, when positioned substantially at the surface of said liquid medium, for measuring second pressure parameters at said surface of said liquid medium and for facilitating the sensing of said second pressure parameters by said differential pressure sensor means; ambient temperature measurement means for measuring ambient temperature near said container and reporting date indicative of said ambient temperature to said computing means; wherein said computing means is configured for recording in a time continuum basis data from said differential pressure sensor means, said high pressure measurement means, said low pressure measurement means, said barometric pressure measuring means, and said ambient temperature measurement means, and for deriving data indicative of changes over time of the mass of said liquid medium within said container.
 2. The system of claim 1 further comprising barometric pressure measuring means configured for measuring atmospheric pressure near said container and reporting date indicative of said atmospheric pressure to said computing means, and wherein said computing means is further configured for deriving said data indicative of changes over time of the mass of said liquid medium within said container, basing such derivation, in part, on said date indicative of said atmospheric pressure.
 3. The apparatus of claim 1 further comprising temperature management means configured for maintaining the temperature of said differential pressure sensor means at a substantially constant level.
 4. The apparatus of claim 2 further comprising temperature management means configured for maintaining the temperature of said differential pressure sensor means at a substantially constant level.
 5. A system for detecting leaks in a liquid storage tank comprising: differential pressure sensor means having a low pressure measurement component and a high pressure measurement component, said high pressure measurement component being configured for measuring tank bottom pressure within a liquid, said low pressure measurement component being configured for receiving data indicative of atmospheric and vapor pressure substantially at the surface of said liquid; barometric pressure measuring means configured for measuring atmospheric pressure substantially at the surface of said liquid; barometric pressure and differential pressure calculation means configured for receiving first and second data reflecting, respectively, said tank bottom pressure and of said barometric pressure substantially at said surface of said liquid, and for adjusting said first data to substantially eliminate variations upon said measurements of said tank bottom pressure caused solely from atmospheric pressure variations to yield an atmospheric pressure adjusted tank bottom pressure; ambient temperature measurement means for measuring ambient temperature near said storage tank; tank dynamic and barometric pressure adjusted tank bottom pressure calculation means configured for receiving third data indicative of said atmospheric pressure adjusted tank bottom pressure, for receiving fourth data indicative of ambient temperature measurements by said ambient temperature measurement means, for receiving fifth data indicative of expansion characteristics of said storage tank, and for adjusting said third data with reference to said fourth and fifth data to substantially eliminate variations upon measurements and calculations of said barometric pressure adjusted tank bottom pressure, caused solely by dimensional changes in said storage tank resulting from atmospheric temperature variations, to yield a tank dynamic adjusted tank bottom pressure; tank content mass calculation means for calculating mass contents of said storage tank substantially based on said tank dynamic adjusted tank bottom pressure and operator input data reflecting physical characteristics of said contents of said storage tank.
 6. The apparatus of claim 5 further comprising temperature management means configured for maintaining the temperature of said pressure transducer substantially at an operator-specified temperature.
 7. A method for detecting leaks in a storage receptacle; comprising the steps of: selecting a mass detection system comprising: differential pressure sensor means having a low pressure measurement component and a high pressure measurement component, said high pressure measurement component being configured for measuring tank bottom pressure within a liquid, said low pressure measurement component being configured for receiving data indicative of atmospheric and vapor pressure substantially at the surface of said liquid; barometric pressure measuring means configured for measuring atmospheric pressure substantially at the surface of said liquid; barometric pressure and differential pressure calculation means configured for receiving first and second data reflecting, respectively, said tank bottom pressure and of said barometric pressure substantially at said surface of said liquid, and for adjusting said first data to substantially eliminate variations upon said measurements of said tank bottom pressure caused solely from atmospheric pressure variations to yield an atmospheric pressure adjusted tank bottom pressure; ambient temperature measurement means for measuring ambient temperature near said storage tank; tank dynamic and barometric pressure adjusted tank bottom pressure calculation means configured for receiving third data indicative of said atmospheric pressure adjusted tank bottom pressure, for receiving fourth data indicative of ambient temperature measurements by said ambient temperature measurement means, for receiving fifth data indicative of expansion characteristics of said storage tank, and for adjusting said third data with reference to said fourth and fifth data to substantially eliminate variations upon measurements and calculations of said barometric pressure adjusted tank bottom pressure, caused solely by dimensional changes in said storage tank resulting from atmospheric temperature variations, to yield a tank dynamic adjusted tank bottom pressure; tank content mass calculation means for calculating mass contents of said storage tank substantially based on said tank dynamic adjusted tank bottom pressure and operator input data reflecting physical characteristics of said contents of said storage tank; selecting data storage means for collecting a plurality of mass content data as generated by said tank content mass calculation means over a plurality of points in time; selecting computing means configured for generating a human perceptible indication of changes in said mass content data between a plurality of said points in time; actuating said mass detection system; and recording data indicative of changes in said mass content data attributable to leakage of said storage tank to detect of such leakage.
 8. The method of claim 7 further comprising the steps of: securing all input and outflow orifices of said storage tank before said actuation of said mass detection system; and substantially selectively processing said mass content data which were generated approximately between sunset and sunrise at an installation site of said system.
 9. The apparatus of claim 1 further comprising protective enclosure means configured for maintaining therein one or more components of said apparatus, other than said low pressure and high pressure components, substantially at a vacuum.
 10. The apparatus of claim 1 further comprising protective enclosure temperature management means configured for maintaining the temperature of said differential pressure sensor substantially at a operator-specified temperature.
 11. A method for detecting leaks in a storage receptacle; comprising the steps of: selecting a mass detection system comprising: differential pressure sensor means having a low pressure measurement component and a high pressure measurement component; protective enclosure means for protecting said differential pressure sensor means from materials and conditions when immersed in liquid contents of said storage tank; said high pressure measurement component being configured for measuring tank bottom pressure within a liquid in which said canister is immersed; said low pressure measurement component being configured for receiving data indicative of atmospheric pressure substantially at the surface of said liquid barometric pressure measuring means configured for measuring atmospheric pressure substantially at the surface of said liquid when said protective enclosure means is immersed therein; barometric pressure and differential pressure calculation means configured for receiving first and second data reflecting, respectively, said tank bottom pressure and of said barometric pressure substantially at said surface of said liquid, and for adjusting said first data to substantially eliminate variations upon said measurements of said tank bottom pressure caused solely from atmospheric pressure variations to yield an atmospheric pressure adjusted tank bottom pressure; ambient temperature measurement means for measuring ambient temperature near said storage tank; fluid temperature measurement means for measuring fluid temperature inside the storage tank; tank dynamic and barometric pressure adjusted tank bottom pressure calculation means configured for receiving third data indicative of said atmospheric pressure adjusted tank bottom pressure, for receiving fourth data indicative of ambient temperature measurements by said ambient temperature measurement means, for receiving fifth data indicative of fluid temperature measurements by said fluid temperature measurement means, for receiving sixth data indicative of expansion characteristics of said storage tank, and for adjusting said third data with reference to said fourth, fifth and sixth data to substantially eliminate variations upon measurements and calculations of said barometric pressure adjusted tank bottom pressure, caused solely by dimensional changes in said storage tank resulting from atmospheric temperature variations, to yield a tank dynamic adjusted tank bottom pressure; tank content mass calculation means for calculating mass content data representative of the contents of said storage tank substantially based on said tank dynamic adjusted tank bottom pressure and operator input data reflecting physical characteristics of said contents of said storage tank; selecting data storage means for collecting a plurality of mass content data as generated by said tank content mass calculation means over a plurality of points in time; selecting computing means configured for generating a human perceptible indication of changes in said mass content data between a plurality of said points in time; placing said protective enclosure means substantially at a bottom interior surface of said storage tank; actuating said mass detection system; and observing data indicative of changes in said mass content data attributable to leakage of said storage tank to detect of such leakage. 